1. Field of the Invention
The present invention relates to a metal separator for a PEFC (Polymer Electrolyte Fuel Cell).
2. Description of Related Art
A PEFC apparatus includes individual fuel cells. Each fuel cell includes a membrane-electrode assembly (MEA) and a separator. The MEA includes an electrolyte membrane and a pair of electrodes disposed on opposite sides of the electrolyte membrane. The pair of electrodes includes an anode provided on one side of the membrane and constructed of a first catalyst layer and a cathode provided on the other side of the membrane and constructed of a second catalyst layer. A first diffusion layer may be provided between the first catalyst layer and a first separator and a second diffusion layer between the second catalyst layer and a second separator. The first separator has a passage formed therein for supplying fuel gas (hydrogen) to the anode and the second separator has a passage formed therein for oxidant gas (oxygen, usually, air) to the cathode. A plurality of fuel cells are layered to construct a module. A number of modules are piled, and electrical terminals, electrical insulators, and end plates are disposed at opposite ends of the pile of modules to construct a stack of fuel cells. After tightening the stack of fuel cells between the opposite end plates in a fuel cell stacking direction, the end plates are coupled to a fastening member (for example, a tension plate) extending in a fuel cell stacking direction outside the pile of fuel cells by bolts extending perpendicularly to the fuel cell stacking direction.
In the PEFC, at the anode, hydrogen is changed to positively charged hydrogen ions (i.e., protons) and electrons. The hydrogen ions move through the electrolyte to the cathode where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of the adjacent MEA and move to the cathode of the instant MEA through a separator) to form water as follows:
At the anode: H2xe2x86x922H++2exe2x88x92
At the cathode: 2H++2exe2x88x92+(xc2xd)O2xe2x86x92H2O
To cool the fuel cells, the temperature of which rises due to the heat generated at the water production reaction and a Joulean heat, a cooling water passage is formed at every cell or at every module and a cooling water is caused to flow in the cooling water passage.
In a case where a length of the reactant gas passage is short, a gas flow speed required for causing a necessary amount of gas to flow over a predetermined MEA cross-sectional area is low, so that diffusion of gas to the electrode is slow, resulting in lower fuel cell power output. Further, when the gas speed is low, the product water is unlikely to be conveyed to the outlet of the gas passage by the gas flow, so that the product water is apt to stay in the gas passage to block the gas passage.
In order to make the length of the reactant gas passage long to thereby raise the power of the fuel cell and to suppress retention of the product water, usually the gas passage is constructed serpentine.
Japanese Patent Publication No. 2000-228207 discloses a metal separator having a serpentine gas passage. The separator includes two outside plates and an intermediate plate disposed between the two outside plates. A serpentine gas passage is formed in each outside plate at the outside surface of the outside plate. A cooling water passage is formed between the outside plate and the intermediate plate and is serpentine corresponding to the serpentine structure of the gas passage. Therefore, the cooling water passage is long like the gas passage.
However, with the conventional metal separator, there are the following problems:
First, since the cooling water passage is serpentine and long, the temperature of the cooling water is considerably high near an outlet of the cooling water passage so that cooling of the fuel cell is not effective.
Second, since the cooling water passage is serpentine, the pressure loss at the cooling water passage is large.
Third, if only the gas passage is made serpentine and the cooling water passage is made straight, at an intersection of both passages, the cooling water passage is blocked by a wall of a U-tern portion of the gas passage. Therefore, the cooling water passage cannot be designed straight.
An object of the present invention is to provide a separator for a fuel cell where a cooling efficiency of the fuel cell is raised, maintaining a good power characteristic.